The invention relates to light conducting glass fibres and more particularly to the production of glass used in the preparation of preforms for optical fibres. Numerous optical fibres production processes are known in the art.
Certain of them, like the process called "modified chemical vapour deposition" or M.C.V.D, produce fibres with very high performance levels but which are expensive, because the deposition speeds are low. Others like "grain by grain" plasma deposition have high deposition speeds, but do make it possible to produce very high performance fibres (very low attenuation, linked with very high pass bands) due to the difficulties encountered in producing highly doped glass by this method.
French Pat. No. 2,368,444, relating to the so-called "separation of phases and washing" or S.P.L. process, filed by the present Applicant, describes a process for the separation of the phases and for the acid washing or leaching of the glass in order to produce optical fibre preforms made from very high purity glass. The S.P.L. process is used for producing glass fibres with a radial refractive index gradient involving the prior production of a blank having a diameter greater than that of the fibre, as well as the following stages:
a first stage consisting of mixing the starting substances liable to be involved in a glass composition having the separation phenomenon in two solid interconnected and continuous phases of different compositions, the said starting substances having an impurity level below 10.sup.-5, calculated by taking account of the molar proportions of the so-called transition metal oxides, said first stage then involving the preparation of a molten glass bath; PA1 a second stage involving a first stage of drawing the blank at a predetermined speed from the glass bath through a cooling device having a predetermined thermal gradient, and a third stage involving at least one thermal annealing process; and PA1 a another stage involving at least one washing or leaching operation to eliminate the phase containing most impurities, as well as a thermal drying and consolidation treatment of the blank. PA1 a first stage of melting a doped composition, for example, a doped borosilicate composition, having a mixture of oxides and carbonates, whose initial composition makes it possible to induce, by annealing, a separation of phases with an interconnected structure due to spinodal decomposition and of refining the said glass in order to obtain a homogeneous, bubble-free glass; PA1 a second stage of controlling the initial phase separation; PA1 a third stage of annealing the said glass in order to induce the separation of the phases by spinodal decomposition, which then makes it possible to obtain a hard phase formed from silica and at least one doping oxide and a soft phase composed of oxides including at least boron and sodium oxides, the two phases being interconnected; PA1 a fourth stage of etching the said glass with basic acid mixtures or solutions, which may contain salts or complexing agents in order to dissolve the soft phase and obtain a porous glass essentially constituted by the hard phase; PA1 a fifth stage of rinsing said porous glass with distilled water in order to eliminate elements from the soft stage; PA1 wherein in the second stage of controlling the initial phase separation the glass is formed or shaped and hardened in order to control the initial phase separation by pouring it into moulds or onto plates, whereby said process also involves, PA1 a sixth stage of drying the glass by slowly raising the temperature to that making it possible to retain an open porosity 60% higher than the initial porosity; PA1 and a seventh stage of producing a powder from said porous glass.
One of the limitations of this process is that the glass blanks to a certain degree break in the case of acid washing. Another limitation is that it is difficult to produce optical sheets in a reproducible manner by the chemical method.